This invention concerns a method to produce strip, starting from thin slabs produced by a continuous casting plant.
The invention concerns also a plant to produce strip, starting from thin slabs, the plant being suitable to carry out the above method.
The state of the art includes various types of plants to produce strip, starting from thin slabs produced continuously by continuous casting, but all these plants entail a series of problems which have still not been overcome.
Some plants of the state of the art, which we shall call type "A" for simplicity, tend in particular to roll the thin slab with thicknesses, when entering the rolling train, which are greater than the conventional thicknesses so as to be able to save energy in the heating of the thin slab.
Other plants, which we shall call type "B" for simplicity, tend to introduce a direct rolling process with one or more roughing rolling mill stands immediately downstream of the casting plant and upstream of the shearing step so as to produce as output from the roughing rolling mill stands a bar having a thickness within the traditional range of 18 to 35 mm. and even less if a reel in the hot state is included on which the bar is wound.
In such plants the thin slab with a thickness of about 40-60 mm. is rolled on one or more roughing rolling mill stands, which work coupled to, and immediately downstream of, the continuous casting plant.
Next, the bar with a thickness of about 18-35 mm. is sheared and wound in a coil and is then unwound and sent to the finishing train.
These systems provide the advantage that the finishing train works under proper known conditions inasmuch as it is working on a bar of a conventional thickness.
As against this, the system entails a plurality of drawbacks due to the very low rolling speed in the roughing rolling mill stand or stands.
To be more exact, the very low rolling speed leads to the creation of pyrocracks in the processing rolls and to the formation of scale in the roughing rolling mill stands owing to the high temperature and low speed.
These plants therefore involve the risk that the scale may be impressed on the processing rolls and thus may reduce their working life and efficiency considerably.
Moreover, the systems to oscillate the moulds for thin slabs are designed to keep the functioning of their oscillation constant.
Furthermore, the methods of the state of the art tend to make as short as possible the time of transfer of the thin slab from the continuous casting plant to the rolling mill so as to reduce the stay times and therefore the losses by oxidation.
This approach, which is now deeply entrenched in this highly specialised field of technology, on the one hand tends to save energy but on the other hand entails an unsatisfactory finished product. The reasons why the finished product is unsatisfactory are manifold.
The high casting speed with thin slabs having a thickness of 40-60 mm. in plants with the normal surfaces of the meniscus in the crystalliser make difficult a proper melting of the powders and therefore make unsatisfactory the lubrication which the molten powders should perform, so that the surface of the thin slab is impaired accordingly.
The method of working of the oscillation units normally employed in the moulds for thin slabs does not adapt the type of oscillation to the specific and properly timed requirements of the thin slab being formed.
This situation prevents a correct linear release from the wall of the crystalliser and has an unfavourable effect on the surface of the thin slab.
The swift progress of the thin slab in the temperature-maintaining furnace does not enable the surface layer of the thin slab to be affected favourably in a desired manner.
The reduced speeds of the roughing rolling have an unfavourably effect on the surface of the finished product since hard fragile scale forms.
In plants of type "A" the use of a great thickness (normally 45 mm. or more) of the bar entering the finishing train leads, on the one hand, to a greater production of scale and, on the other hand, to the production of scale with a high content of Fe.sub.3 O.sub.4 or Fe.sub.2 O.sub.3.
This is due to the fact that the surface temperature of this type of bar during rolling is always high and, even when it is brought down to lower values (for instance, by descaling with jets of water), the great reserve of heat within the bar and the low rolling speed take the surface of the bar quickly back up to a high temperature at which the scale is produced swiftly and is produced with hard oxides.
This scale not only creates surface faults on the strip being produced but also leads to a great abnormal wear of the rolling rolls.
It is known that, if the surface temperature of a bar is kept between a minimum temperature of 700.degree. C. and a maximum temperature of 930.degree. C., the oxides that form consist mainly of FeO, while the percentages of Fe.sub.3 O.sub.4 and Fe.sub.2 O.sub.3 are very low.
FeO produces a malleable scale that can be rolled and forms a continuous and substantially even film, which does not break readily and therefore substantially does not cause problems and does not produce surface faults during subsequent rolling of the bar in the finishing train.
Instead, Fe.sub.3 O.sub.4 and Fe.sub.2 O.sub.3 are very hard, fragile oxides which, when rolled, break in an uneven manner and cause wear in the processing rolls and surface faults, which lower the quality of the strip produced.
In plants of type "A" the thermal capacity of the bar being rolled is such that the heat released from the core of the bar has enough time to bring the surface temperature of the bar above the optimum value of 930.degree. C.
FIG. 1a is a diagram of a finishing train of a known plant producing strip from thin slabs, the train comprising in this case six finishing stands, with an emergency shears and a descaling unit positioned upstream of the train.
In this plant the bar reaches the inlet of the finishing train with a thickness of at least 40 mm. and with an intake speed between about 20 and 35 metres per minute.
FIG. 1b is a diagram of the development of the surface, internal and mean temperatures respectively of the bar in the finishing train of FIG. 1a.
It is clear that the surface temperature at several measurements is above 930.degree. C., thereby generating scale consisting of Fe.sub.3 O.sub.4 and Fe.sub.2 O.sub.3.
FIG. 1c shows a plant of the type "A", in which can be seen, in sequence and in a rough sketch, a mould with a relative continuous casting plant for thin slabs, a shears which shears to size the thin slab leaving the continuous casting plant, a heating furnace five times as long as the length of the sheared slab, a shears for service shearing, a finishing train consisting of six four-high rolling mill stands and a finishing treatment segment.
Normally in plants of type "A" the thin slab reaches the finishing rolling mill stands with a thickness of 50-55 mm.
This type of plant, besides the defects already disclosed, works badly since the rolling mill stands work at a low speed and the danger of growth of scale is constant and great.
FIG. 1d shows a plant of the type "B", in which can be seen, in sequence and in a rough sketch, a mould with a relative continuous casting plant for thin slabs, this continuous casting plant including a treatment of soft-reduction of the thin slab, two roughing rolling mill stands, an emergency shears, a winding unit, another emergency shears, a finishing assembly consisting of four rolling mill stands and a finishing treatment segment.
In this type of plant, besides the defects already disclosed, the roughing rolling mill stands receive a thin slab with a thickness of about 40-45 mm. and therefore work very badly.
U.S. Pat. No. 5,235,840 arranges that a continuous, intense removal of scale is carried on so as to eliminate from the surface of the bar the hard scale of iron oxides so that such scale does not affect the final result of the rolling.
Such a lay-out is shown in FIG. 2a and the relative diagram of the temperatures is shown in FIG. 2b. This teaching provides for elimination of the scale, which forms after the conventional descaling unit, immediately upstream of the rolling mill stands of the finishing train by means of appropriate auxiliary descaling units.
Without these descaling units the hard scale detached unevenly would generate anomalous wear of the rolling rolls and, in some cases, would remain pressed into the surface of the strip.
These intermediate descaling units cool, meanwhile, the surface of the bar being fed, but owing to the low speed of feed in the space between the descaling unit and the relative rolling mill stand the surface temperature rises again to values very close to the physiological limit of 930.degree. C. This has the effect that there is the risk that scale of very hard oxides is produced on the surface of the bar before the rolling performed by the finishing stand.
In fact, these plants entail the problem that the slab enters the finishing rolling mill at a very low speed, which produces an outgoing bar with a poor surface quality, and the bar may have the scale impressed on its surface.
Moreover, these plants have a very limited storage capacity, which may create problems if the line downstream of the storage has to be halted, for instance by a breakdown or for maintenance.
The small size of the storage area causes the need, where the downstream line is halted, to send for scrap the slabs produced by the continuous casting plant or else to halt the continuous casting plant itself.